1 Introduction

To set up the modflow models in the Wierden area, different types of data are required:

Elevations of the different formations
Hydraulic properties of the different formations
Water management of the area
Open water for discharging
- river dimensions (width, side slopes, length profiles(verhang))
- river bed resistance
- weir levels
- Discharge volumes of water
Ditches and (tile)drains for draining agricultural and urban area’s
Recharge stationary and transient
Boundary conditions at the perimeter of the models domain

Steps 3 until 7 will be described in other documents.

Here the focus will be on the elevation of the different formation layers and the hydraulic properties (HK, HV or ratio) of these (combined) formations.

These data are based on LHM (Landelijk Hydrologisch Model) version 4.3. maintained at web page LHM data .

1.1 Elevations of the different formations

With the help of sub soil models region Wierden the following cross section was created:

cross section Wierden area

As shown in ‘grey’ the ice pushed ridge is described being a “gestuwde afzettingen, complexe eenheid”. No hydraulic conductivity or resistance is assigned to this formation.

To get proper elevations from LHM data is a bit tricky!

There are 8 layers in the LHM model .

The layers are defined with top_elevations and bottom_elevations.

Top elevations are:

“top_impermeable_layer1”,

“top_impermeable_layer2”,

“top_impermeable_layer3”

till “top_impermeable_layer8”

Bottom elevations are:

“base_impermeable_layer1”,

“base_impermeable_layer2”,

“base_impermeable_layer3”,

till “base_impermeable_layer8”

One would expect that the top of impermeable layer 2 coincides with the bottom of impermeable layer 1. This is in good agreement most of the time. Some deviations were noticed. Top 3 vs base 2 do differ in the middle somewhat about 3 to 5 m over a short range.

Most striking is the difference between “base_impermeable_layer8” and “geohydrological base”. Not the clip below:

base vs bot last (8) layer

I have no idea why this is!

Also tried to overlay the cross-section of the elevations (Qgis) with the formations from REGIS 2.3, see the clip below

Now considering another approach.

1.1.1 Complex gestuwde formatie becomes formation with low K’s

Through the LHM data site; https://data.nhi.nu/bekijk downloaded the following sets:
horizontale_anisotropie_top.nc
horizontale_anisotropie_bot.nc

With this the ice pushed ridges can be simulated with relative low conductivities mimicken the anisotropical nature of the tilted formations. How derive a proper K is not that easy since the formations are not only clayey deposits.

With knowing the position and depth of the ridges we can now determine three formations to model within modflow using the following elevation data sets:

surface elevations -> “Lagenmodel_LHM43-top_impermeable_layer_1”
top_ridges -> “horizontale_anisotropie_top”
bot_ridges -> “horizontale_anisotropie_bot”
hydrogeological base -> “Lagenmodel_LHM43_base_impermeable_layer_8

The elevation of the hydrogeological base nicely aligns with the top of the Breda formations which are clay deposits.

See the following clip:

As shown in the above clip, the ridges are only very locally present.

1.1.2 Building model layers

MODFLOW-NWT, 2005

This means that for models based on modflow nwt or 2005 that model layers are continous in the whole domain. Basically in the domain all model layers will be present.

Now still two options for layers where the ridges are not present;

thin layers, pinching out till the perimeter
evenly distributed thicknesses of the layers (standard option used till now)

option 1 can give problems being too thin but also issues can arise dry-wet issues. For example too thin layers where RCH need to be extracted can result in problems drying out all cells at that point.

option 2 is most rubost but miss partly ridges when the center of a model not coincides with the ridge.

1.1.2.1 Creating solids

To create new solids for the creation of mf-nwt models the following recipe could work

Required are all elevations; surface, top-ridge, bot-ridge, geohydrological-base

Create a TIN
1. new coverage, large polygon extending the surface_elevation.tif but keep in within avoiding interpolation issues later on
2. set vertices on the polygon arc to 100 m
3. create the TIN by following the steps in the clip:
4. Map the elevations (i.e. the raster files ) to the individual TINs
5. Make sure that the numbering of the TIN’s (mandatory) counts from bottom to top. So geohydrological base receives TIN number 1:
6. The next TIN is not the bottom of the ice pushed ridges which is only partly present. TIN nr. 2. The distribution can be seen in “bot_ridges.tif”):
7. The top of the ice pushed ridges (“top_ridges.tif”)
8. The upper TIN is nr. 4 and is based on “surface_elevation.tif”
9. Create solids from these TIN (horizons):
10. Define the top and bottom TIN elevation for the solids:
11. The following choices were used: Natural neigbor with constant nodal function to avoid strange elevations. “Intersect horizon surfaces” according to the Help; “Allows the solids to intersect with horizon surfaces”. In this case hardly noticeable differences.
12. The solids 3 in this case should appear:
13. As an alternative I tried to use the “top_imp_layer_5.tif” which coincides with the bottom of the ice pushed ridges. With this the idea is that one distinguishes the upper 4 and lower 4 layers of LHM4.3. This resulted in the following solids:
  
  Now the ice-pushed ridge formation is extended way too much.
  
  MODFLOW 6

Since modflow 6 is based on UGRIDs, it is not required to have model layers continuous in the model domain.

1.2 Hydraulic properties of the different formations

The Holterberg is a dominant (hydro)geological feature in the Wierden area. This ice pushed ridge and other hillish-elevated areas, originates from the “Saalien” iceage about 150.000-200.000 yeas ago.

During that time land ice coming from Scandinavia pushed forward the formations which were deposited earlier. These mainly contain sandy, gravely and clayey deposits. As a consequence these formation were pushed forward and sideways resulting tilted formations. The tilted clayey formation within the ice puschd ridge result in anisotropical properties, which means that groundwater flow will encounter different hydraulic conductivities based on the direction of the formation of these ice pusched ridges.

As a consequence the Holterberg and south-eastern ridges from Holterberg need anisotropical parameters for the model layers where they reside.

For the usual set up of a groundwater model in the Wierden we will simply the effect of anisotropical features to a simple resistance.